Separation of acetylene and ethylene from cracked gas

ABSTRACT

Joint separation of acetylene and ethylene from cracked gas, obtained by submerged-flame methods, by condensing the hydrocarbons in one or more stages by cooling to a temperature above the solidification temperature of acetylene in admixture with the other condensable constituents, separating the condensate, washing the residual gas mixture with liquid ethylene and/or ethane, and further cooling the gas mixture free from acetylene to -140* to -170* C.

United States Patent 9 lnventors Appl. No.

Filed Patented Assignee Priority SEPARATION OF ACETYLENE AND ETHYLENE Primary Examiner-Norman Yudkoff Assistant ExaminerArthur F. Purcell Attorney-Johnston, Root, OKeetfe, Keil, Thompson &

Shurtleff c n COOLED mo GAS EXPANDED PORTION 0 H4 ens PORTION FROM CRACKED GAS 4 Claims, 1 Drawing Fig.

U.S. Cl 260/679, 62/24, 62/39, 260/683 Int. Cl F23j 3/02, F25j 3/06, F25j 3/08 Field of Search 62/23, 24, 27, 28, 39; 260/683, 679

/ f :n :"ifio ib n gu l 2 5 cg zxe o uuconosuseo '9 ans PORTION FLASHED CONDENSATE FLASHED couozusnrs C2 H FREE GAS UNCON DENSED GAS PORTION SEPARATION OF ACETYLENE AND ETHYLENE FROM CRACKED GAS This invention relates to a process for the production of acetylene and ethylene from cracked gas.

U.S. Pat. No. 3,471,584 discloses a process for the separation or" acetylene and ethylene from cracked gas also containing methane, ethane and hydrocarbons having from three to five carbon atoms with or without carbon oxysulfide and carbon disulfide, which gas has been obtained in the cracking of petroleum or petroleum fractions by a flame burning beneath the surface of the same and from which carbon dioxide and hydrogen sulfide have been removed, wherein the cracked gas is dried, part of the methane and the whole of the remaining hydrocarbons and any carbon oxysulfide and carbon disulfide are separated by condensation, the condensate is vaporized and compressed and then the acetylene and ethylene together with methane and ethane are separated by pressure distillation from a liquid fraction which contains the whole of the hydrocarbons having three to five carbon atoms and any carbon oxysulfide and carbon disulfide.

In carrying out this process it has been found that under certain conditions, for example when the residual gas from the separators is too high in acetylene, acetylene may be deposited in solid form during further cooling, for example in the flash turbines. Occasional thawing of the turbines is thus made necessary and possibly also of the downstream heat exchangers. Continuous operation may thus be disturbed.

We have now found that disturbances in continuous operation due to deposition of acetylene are avoided when the condensation of the hydrocarbons is subdivided, the hydrocarbons first being cooled to a temperature which is above the solidification temperature of acetylene in admixture with the other condensable constituents, i.e., to from 82 to 1 20 C.,

the condensate thus obtained being separated and the remaining gas mixture containing small amounts of acetylene being freed from acetylene by scrubbing with a liquid consisting of ethylene, ethane or a mixture thereof and the gas mixture devoid of acetylene then being cooled to 1 to 170 C.

In the process according to this invention the above-mentioned disadvantage does not occur because the acetylene content is decreased to such an extent by subdivision of the condensation and the ethylene wash that solid acetylene is not deposited.

The new process is used to process gases that have been obtained in the submerged flame method of cracking liquid hydrocarbons, particularly crude oils, high-boiling crude oil fractions, distillation residues, or mixtures thereof. Such gases contain, in addition to ethylene, acetylene, hydrogen, nitrogen, carbon monoxide, carbon dioxide, methane ethane and higher hydrocarbons with from three to six carbon atoms, sulfur compounds such H 8, COS and CS When the gas is condensed to -82 to l 20 C., the C to C hydrocarbon fraction containing COS and CS and the bulk of the acetylene, e.g., to 95 percent of the total amount of acetylene present, are separated. The condensate additionally contains 30 to 80 percent of the ethylene and ethane and small amounts of undissolved methane. The condensation may if desired by carried out in two or more steps.

The residual gaseous portion is washed with ethylene or ethane or a mixture thereof. A mixture of ethylene and ethane such as is obtained in the further separation is preferred.

After the residual acetylene has been thus removed from the cracked gas the gas is further cooled to l40 to-l 70 C. to remove the residual ethylene and. ethane.

In carrying out the process, the necessary preliminary separation of the carbon dioxide and any sulfur, for example in the form of hydrogen sulfide, may be carried out in known manner, for example by treatment with an aqueous solution of an alkali metal salt of an amino acid. Methods referred to in the main patent may generally be used for this purpose. This is also true of any fine purification from carbon dioxide and subsequent drying of the cracked gas devoid of carbon dioxide and hydrogen sulfide prior to condensation. In addition to the drying specified in the main patent, scrubbing with anhydrous methanol or saturation with methanol vapor has proved to be suitable. I

The condensation itself is generally carried out at a pressure of from 5 to .40 atmospheres. A particular embodiment of the process consists in raising the temperature of the gas (cooled to -l40 to l70 C. after the ethylene wash) after condensate has again been separatedto to l00 C. countercurrent to the gas leaving the ethylene scrubber. The gas mixture which has had its temperature raised in this way may be flashed in a flash unit with performance of external work so that the gas mixture is cooled and may be used for countercurrent cooling in the condensation stages.

For scrubbing the gas mixture after it has been cooled to 82 to l20 C., it is possible to use the condensate which has been obtained by cooling to to 170 C. after the scrubbing with ethylene. It is also possible to add to this liquid additional amounts of ethylene and/or ethane obtained from other sources.

The condensate deposited in the condensation stages or if desired, in a single condensation stage may be flashed and vaporized in countercurrent heat exchange for cooling the gas mixture.

The invention is illustrated by the following example.

EXAMPLE I by volume H, 29.37 N, 059 CO 40. l 0 O, 0.20 CH. 3.96 C,H, 6.46 C H, 0.49 C l-l, 3.84 C,H,, 246 CH 4.89 C H, 6.95 CH,OH 0.69.

The gas is cooled to 20 C., the condensate obtained being separated in a separator 3. The remaining gas is passed through line 4 to a heat exchanger 5, cooled to 110 C. and freed from condensate in a separator 6. The residual gas flows through line 7 into a column 8 in which by countercurrent scrubbing with a mixture consisting substantially of liquid ethylene and ethane it is freed from acetylene to such an extent that the acetylene content in line 9 is less that 1 percent (with reference to the mixture of acetylene and ethylene) so that upon further cooling to C. in a heat exchanger 10 no deposition of solid acetylene can take place. Gas and liquid are separated in a separator 11. The gas with a residual content of ethylene of 2,000 p.p.m. and of acetylene of less than 10 p.p.m. passes in line 12 through the heat exchanger 10 and is warmed to a temperature of 125 C. countercurrent to the current of gas in line 9 and supplied to a flash turbine 13. Here the gas is expanded from 7.8 atmospheres to 1.4 atmospheres while performing external work, its temperature thus being lowered to 1 69 C.

We claim:

1. In a method of joint separation of acetylene and ethylene from cracked gas which has been obtained by cracking liquid hydrocarbon fractions by means of a flame burning beneath the surface of said hydrocarbons from which the carbon dioxide and hydrogen sulfide have been removed and the cracked gas is dried by condensation the improvement which comprises cooling said hydrocarbons to a temperature which is above the solidification temperature of acetylene in admixture with the other condensable constituents and which is in the range of82 to 1 20 C., the condensate thus obtained being separated and the remaining gas mixture containing small amounts of acetylene being freed from acetylene by scrubbing with a liquid consisting of ethylene, ethane or a mixture thereof, the gas mixture devoid of acetylene then being cooled to from l40 to 170 C., and flashing and vaporizing the condensate separated in the two condensation stages in countercurrent heat exchange for cooling the inflowing cracked gas.

2. In a method of joint separation of acetylene and ethylene from cracked gas which has been obtained by cracking liquid hydrocarbon fractions by means of a flame burning beneath the surface of said hydrocarbons from which the carbon dioxide and hydrogen sulfide have been removed and the cracked gas is dried by condensation the improvement which comprises cooling said hydrocarbons to a temperature which is above the solidification temperature of acetylene in admixture with the other condensable constituents and which is in the range of 82 to 1 20 C., the condensate thus obtained being separated and the remaining gas mixture containing small amounts of acetylene being freed from acetylene by scrubbing with a liquid consisting of ethylene, ethane or a mixture thereof, the gas mixture devoid of acetylene then being cooled to from to l70 C., separating the condensate from the gas which has been cooled to from l40" to l70 C., warming said gas to l30 to l0O C. countercurrent to the gas leaving the ethylene scrubber, and flashing said gas in a flash unit while performing external work so that the gas mixture is cooled and is used for countercurrent cooling in the condensation stages.

3. In a method as claimed in claim 1 the improvement which comprises using the condensate obtained by cooling to from 1 40 to 1 70 C. as the scrubbing liquid.

4. In a method as claimed in claim 2 the improvement which comprises using the condensate obtained by cooling to from 140 to C. as the scrubbing liquid. 

2. In a method of joint separation of acetylene and ethylene from cracked gas which has been obtained by cracking liquid hydrocarbon fractions by means of a flame burning beneath the surface of said hydrocarbons from which the carbon dioxide and hydrogen sulfide have been removed and the cracked gas is dried by condensation the improvement which comprises cooling said hydrocarbons to a temperature which is above the solidification temperature of acetylene in admixture with the other condensable constituents and which is in the range of -82* to -120* C., the condensate thus obtained being separated and the remaining gas mixture containing small amounts of acetylene being freed from acetylene by scrubbing with a liquid consisting of ethylene, ethane or a mixture thereof, the gas mixture devoid of acetylene then being cooled to from -140* to -170* C., separating the condensate from the gas which has been cooled to from -140* to -170* C., warming said gas to -130 * to -100* C. countercurrent to the gas leaving the ethylene scrubber, and flashing said gas in a flash unit while performing external work so that the gas mixture is cooled and is used for countercurrent cooling in the condensation stages.
 3. In a method as claimed in claim 1 the improvement which comprises using the condensate obtained by cooling to from -140* to -170* C. as the scrubbing liquid.
 4. In a method as claimed in claim 2 the improvement which comprises using the condensate obtained by cooling to from -140* to -170* C. as the scrubbing liquid. 